Spherical aberration and focusing

[mike_tee_vee]

Can someone please explain to me the effects of spherical aberration on a lens' focusing system?  Several lens tests on the net cite that spherical aberration causes focus errors at certain aperture.

Is this inherent to most lens designs?  Is there a certain range where it is less pronounced?

[Bob Atkins]

Spherical aberration occurs when light passing through the outer part of a lens is brought to a focus point which is different then light passing through the center of the lens. It's really only a significant aberration at large apertures. Once you stop a lens down, the light only passes through the central region and so comes to focus only at one point.

Autofocus syetems always focus with the lens wide open, so that means that spherical aberration will be at its maximum. The problem then is where the exact focus point is. There will be a point at which the blur disk will be a minimum size, but this will likely be slightly further from the lens than the point at which the outer rays come to focus and slightly closer to the lens than the point where the center rays come to focus. If you focus for the minimum blur circle at maximum aperture, you may not have the best focus when the lens is stopped down. This is sometimes called a "focus shift on stopping down".

Very few modern lenses exhibit such a focus shift. The lenses most likely to show it would be very fast lenses (f1.2, f1.0) and even then it might only be noticable at close focus distances where spherical aberration can increase over the level seen at normal focus distances.

[klindup]

I seem to remember that Canon once made an f0.95 lens, which is as far as I know the fastest ever.  Of course it did not rely on autofocus and in fact was not even an SLR, it used a coupled split image rangefinder that relied on parallax.  As an old photography hand I remain to be convinced that autofocus is the be all and end all.

Ken Lindup